EK307 – Electric Circuit Theory – Homework #4 – Fall 2009
Tutorial Problems on Transistors and Diodes
Problem 1
)
The semiconductor diode (sometimes called the
pn
junction diode) is a very common nonlinear
circuit element. The equation for the diode current due to an applied voltage is exponential in
form:
)
1
(
/

=
T
D
V
v
S
D
e
I
i
Here
I
S
is a devicespecific constant called the
scale current
, and
V
T
is the thermal voltage given
by
kT
/
q
.
(1)
Note that
V
T
= 0.025 V at room temperature. This equation is definitely
not
linear
and does
not
have the form
y
=
mx
+
b
.
(Contrast to Ohm’s Law:
i
=
v
/
R.
)
In this problem, we plot the current
i
D
as a function of applied voltage
v
D
.
a) For a diode having
I
S
= 10

12
A, fill in the table to the right, showing the
current
i
D
that results for each value of
v
D
.
b) Now plot these numbers with the
i
D
values from the table on the vertical
axis, and the
v
D
values from the table on the horizontal axis. The
v
D
axis
itself should extend from zero to 5 V. What letter of the alphabet or
geometrical form best described the nature of this curve?
c) We define the value
v
D
=
V
f
as the point where the curve makes its
transition from being essentially horizontal to being essentially vertical.
The voltage
V
f
represents the “threshold” value of
v
D
below which current
will not flow. Estimate the value of
V
f
for this diode.
Problem 2
)
As in Prob. 1, estimate the value of
V
f
for diodes having scale currents of
a)
I
s
= 10

15
A
b)
I
s
= 10

10
A
Problem 3
)
A diode for which
V
f
= 0.7 V is connected to a Thévenin circuit, as shown below.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '09
 Horenstein
 Transistor, Pn junction, Input port

Click to edit the document details